The remote identification of aerosol particles is critical in many different applications. Of primary importance is the remote identification of chemical, radiological or biological agents in the atmosphere. In modern times the potential of intentional releases of a biological, chemical or radiological contaminants into the atmosphere is a real and serious threat to civilian populations in the United States and abroad and also to military personnel throughout the world. It would be extremely beneficial to be able to identify and track such aerosol particles remotely. The identification information could be used to initiate proper safety precautions and to identify the source of the aerosol particles.
Another critical application for remote aerosol particle identification is in the field of environmental science. Knowing the location and composition of aerosol particles can assist in many ways. For example, environmental regulators could detect sources of particulate matter in densities too low for visual identification. By analyzing the motion of the aerosol distribution over time, wind vectors can be determined. This data could then be used to determine possible sources of the aerosol particles along with potential future distribution of the aerosol particles. Environmental regulators could also use aerosol particle identification to determine the composition of plumes emanating from industrial facilities. In the case of a plume that poses an immediate or long-term health risk, the data would be valuable to health officials and researchers. Tracking the behavior of plumes of aerosol particles and determining the constituents of plumes of aerosol particles could also be valuable to meteorologists and researchers studying the effects of pollution on global warming and climate behavior.
Current methods of remotely obtaining information about aerosol particles are limited in their capabilities. Generally, these systems transmit laser energy into the atmosphere and detect returned radiation for analysis to identify particle aggregations and atmospheric structure of interest. The laser energy is transmitted into the atmosphere in the form of laser pulses. After a short laser pulse the system monitors for returned radiation. The delay between the time the pulse was initiated and the time the return radiation was received indicates the distance from the transmitter of the particles that caused the radiation to be returned.
The measurement of depolarization characteristics has been used to determine the state of water clouds in the atmosphere. In this method, linearly polarized light is transmitted into the atmosphere and the backscatter is examined for out-of-plane backscattered radiation. A low amount of out-of-plane backscattered radiation indicates that the water vapor is in liquid form. A higher amount of out-of-plane backscattered radiation indicates that the water particles exist in the form of ice crystals. Known depolarization measurement systems use primary wavelengths significantly shorter than 1.5 microns. Due to the wavelengths used by known systems, this method is either non eye-safe, in that the transmitted optical beams used do not meet critical safety standards and therefore extraordinary safety precautions are required, or the method is used at such a low power as to be eye-safe. However, in this low-power mode the system must time-average the backscatter information over a significant period of time resulting in long measurement times. This is generally achieved by incorporating photon-counting receivers. These long measurement times also prevent known eye-safe systems from scanning the atmosphere in a timely manner. Rapid scanning is important in order to create coherent time-lapse animations of the data, which reveal characteristics of the atmospheric flow and dispersion.